1. Field of the Invention
This invention relates to a panel having an insulating substrate, thin film elements formed by stacking thin films on the substrate, and wires formed to supply signals to the thin film elements.
2. Description of the Related Art
A panel having an insulating substrate, thin film elements formed on the substrate, and wires formed to supply signals to the thin film elements is used for forming, for example, an active matrix liquid crystal display device, etc.
A conventional thin-film-element-formed panel, which is used for forming an active matrix liquid crystal display device having thin film transistors serving as active elements, has its part constructed as shown in FIG. 1.
This panel has, on its transparent insulating substrate 1 made of glass, a plurality of pixel electrodes 2, a plurality of thin film transistors 3 serving as active elements, scanning wires 4 for supplying scanning signals to the transistors 3, and data wires 8 for supplying data signals to the transistors.
As is shown in FIG. 2, each of the thin film transistors 3 is formed by stacking a gate electrode G, a gate electrode insulating film 5 made of SiN (silicon nitride), and an i-type semiconductor layer 6 made of a-Si (amorphous silicon), and further stacking a source electrode S and a drain electrode D on the layer 6 with an n-type semiconductor layers 7, made of a-Si, interposed therebetween.
The gate electrode G of the thin film transistor 3 is formed integral with the scanning wire 4, the drain electrode D is formed integral with the data wire 8, and the source electrode S is connected to the pixel electrode 2. The gate insulating film 5 is formed over the scanning wire 4, and substantially all over the substrate 1. The pixel electrode 2 and data wire 8 are formed on the gate insulating film 5.
The scanning wire 4 includes that portion of the thin film transistor 3 which serves as the gate electrode G, and is made of Cr (chrome), Ta (tantalum), or Ta-Mo (tantalum-molybdenum) alloy. The data wire 8, drain electrode D, and source electrode S are formed of a metal film of Al (aluminum). A contact metal film 9 made of Cr and being in good ohmic contact with the n-type semiconductor layer 7 is provided under the data wire 8, drain electrode D, and source electrode S.
The scanning wire 4 is formed by providing a metal film on the substrate 1 and then patterning the metal film using photolithography. If an exposure mask to be used for exposing a photoresist film formed on the metal film has a defect, the patterned scanning wire 4 may be broken.
This damage may occur irrespective of whether the photoresist is of a positive type or of a negative type. In the case of using a positive photoresist, if the exposure mask has a defect or broken portion, a resist mask obtained by exposing, developing, and patterning a photoresist film will have a broken portion at a location corresponding to that of the exposure mask.
On the other hand, in the case of using a negative photoresist, if dust or the like is attached to a portion of the exposure mask, the resist mask will have a broken portion at a location corresponding to that portion of the exposure mask. In this case, also when dust or the like is attached to a photoresist film formed on the substrate, the resist mask may be broken.
Where the resist mask is broken, that portion of the metal film which is located corresponding to the broken portion of the mask is etched, so that the scanning wire 4 formed by patterning the metal film is inevitably broken.
The same can be said of the data wire 8. Since the wire 8 and the contact metal film 9 located under the former are patterned by photolithography, the patterned data wire 8 and contact metal film 9 may be broken when the exposure mask has a defect.
So as not to cause such a broken portion, in the conventional panel, each of the scanning wire 4 and data wire 8 has a two-layered structure with two metal layers patterned in different processes.
More specifically, as is shown in FIGS. 2-4, the scanning wire 4 comprises a lower metal film 4a and an upper metal film 4b laminated thereon. First, a film is formed and patterned by photolithography using an exposure mask to provide the lower metal film 4a, and then another film is formed on the patterned film 4a and patterned by photolithography using another exposure mask to provide the upper film 4b.
In the scanning wire 4 constructed as above, even if a broken portion A exists in the lower metal film 4a due to a defective portion in the exposure mask, the film 4a can have conductivity via the upper metal film 4b, thereby preventing breakage in the wire.
So with the data wire 8. As is shown in FIG. 2, the wire 8 has a two-layered structure, i.e. consists of a lower metal film 8a and an upper metal film 8b laminated thereon. The films 8a and 8b are patterned by photolithography using different exposure masks in different processes. Thus, even if the lower metal film 8a is broken, the broken portion is covered by the upper metal film 8b, thereby preventing a breakage in the wire.
In the above example, descriptions are made as to a thin-film-element-formed panel to be used for forming an active matrix liquid crystal display device having a thin film transistor serving as an active element. However, the idea of forming a wire of a two-layered structure has been applied also to other types of thin-film-element-formed panels--one to be used for forming an active matrix liquid crystal display device having that thin diode or non-linear resistor element of an MIM (metal film-insulating film-metal film) structure which serves as an active element; another having an electronic circuit comprising thin film elements.
However, in the above-described conventional panel, breakage may be caused in the wire even if the wire has a two-layered structure. This is because a defect may exist in an exposure mask used for exposing a photoresist film when the upper metal film is patterned.
That is, as is described above, in the scanning wire 4 shown in FIGS. 3 and 4, the lower metal film 4a is formed and patterned, and then the upper metal film 4b is formed and patterned on the former. At this time, if there is a defective portion in the exposure mask used for exposing the photoresist film when the upper metal film 4b is patterned, a broken portion B will be caused in that portion of the upper film 4b which corresponds to the defective portion. This broken portion will cause etching of a corresponding portion of the lower metal film 4a. That is, the wire will be broken at B.